This invention relates to a method of manufacturing a pneumatic tire and a vulcanization mold used therefor, and more particularly to a method of manufacturing a pneumatic tire comprising a bead portion provided with a protruding portion of, for example, a trapezoidal shape at its section, which is called as a bead guard, through vulcanization-shaping and a vulcanization split type mold for realizing the vulcanization-shaping in this method. Especially, the invention provides a bead guard showing a good appearance.
In the vulcanization-shaping of a radial ply tire among pneumatic tires, it is general to use a split type mold as a vulcanization mold capable of advantageously coping with a high precision finishing and a variety of tread patterns in the tire. All the tire described hereinafter indicates a radial ply tire.
The split type mold comprises a pair of side rings forming a pair of bead portions and a pair of sidewall portions of a pneumatic tire, and many split segments for the formation of a tread portion of the pneumatic tire each freely going in and out inward and outward in a radial direction with respect to a center axis line of the side ring and engaging with the respective side ring as mentioned later in detail.
An integrally engaged face between the side ring and the segment is existent in a position corresponding to an inward of the tire tread portion in a radial direction. And also, the side ring has an integrally engaged face with a bead ring shaping a part of a bead portion extending outward from a bead base in the radial direction, more concretely up to a neighborhood of a central position of a flange height of an approved rim to be mounted with the tire during vulcanization.
When an uncured tire held in the split type mold is subjected to a vulcanization-shaping, a pressurized gas is filled into an inner face of the uncured tire to push the uncured tire onto the split type mold. As a result, air reservoirs are created between the split type mold and the uncured tire though the number of the air reservoirs is far less than that in a conventional full mold or so-called two-split mold. As the vulcanization proceeds at such a state, a rough face called as bare is formed in a cured tire to remarkably impair an appearance of the tire.
Even in the split type mold, therefore, through-type ventholes for venting air are disposed in many places easily causing the bare. In this case, rubber flowed into an inside of the venthole during the vulcanization renders into many spew rubbers protruded from a surface of a tire after the vulcanization-shaping. Since it is required to remove these spew rubbers at a separate trimming step, there are caused problems that the number of working steps increases and direct material cost and industrial waste cost spoil and the appearance is impaired by trimmed traces.
For this end, it is variously attempted to largely decrease the number of the ventholes or remove them, but complete removal is not still attained.
When a pneumatic tire comprising a bead guard is vulcanization-shaped in the split type mold, an air reservoir is apt to be formed in a depressing portion of the side ring for the formation of the bead guard protruding outward from the tire on a circumference of the bead portion and frequently shaping into a torus.
The reserved air is compressed through rubber by a pressure of a high-pressure gas acting to an inside of an uncured tire. A part of the compressed air is scattered and lost around the depressing portion, while the remainder is particularly crowded between a corner part of the protruded portion like the bead guard at the outer surface of the tire and a bottom corner part of the depressing portion of the side ring corresponding thereto to create the bare in the corner part of the bead guard.
The thus created bare considerably imparts the appearance of the tire but also causes a problem that the bead guard remarkably comes off from an expected shape.
In order to prevent the occurrence of such bears, it is required to dispose many ventholes in the bottom corner part of the depressing portion of the side ring as previously mentioned. In this case, however there are caused the aforementioned problems and also an operation for removing spews from a top corner part of the bead guard is particularly such a delicate operation that damage must not be given to the top corner part and brings in difficulty at the removal through mechanical operation, which is different from the case of removing spews at the other tire positions. Further, since such a protruding portion is easily noticeable, it is apt to remarkably impart an appearance of the tire by traces damaging the top corner parts and spew-removed traces.
Therefore, the invention aims at a pneumatic tire comprising a bead guard in its bead portion and is to provide a method of manufacturing a pneumatic tire capable of forming a bead guard having an excellent appearance without creating bare trouble and spew in the bead guard and a vulcanization mold usable for carrying out this method.
According to the invention, there is a method of manufacturing a pneumatic tire comprising a tread portion, a pair of sidewall portions and a pair of bead portions communicating with both sides of the tread portion, and a bead guard disposed in at least one bead portion and protruding outward from the tire by vulcanization-shaping with a split type mold comprising a pair of annular side rings forming the pair of bead portions and the pair of sidewall portions and many split segments for the formation of the tread portion integrally united and engaged with the respective side ring, characterized in that an edge of a dividing face between mutually adjoining side ring pieces by annularly dividing the side ring inward and outward in a radial direction with respect to a center axis line of the split type mold and located at a side of a tire shaping face is positioned in a place corresponding to a depressing portion forming a bead guard, more preferably a place corresponding to a bottom of the depressing portion.
It is favorable that the edge of the dividing face between the mutually adjoining side ring pieces located at the side of the tire shaping face is positioned in a place corresponding to a corner part of the depressing portion forming the bead guard.
When an uncured tire is pushed onto a shaping face of the mold, air is easily escaped into such a place that a flow-displaced or deformed rubber of the uncured tire arrives at the shaping face of the mold at a rate slower than that of the other rubber portion and hence the bare is easily caused in such a place, so that when there are plural corner parts, it is favorable to position the edge of the dividing face between the mutually adjoining side ring pieces located at the side of the tire shaping face in a place corresponding to a deeper corner part.
On the other hand, when the uncured tire is gradually enlarged and deformed from inward to outward in the radial direction by using, for example, a shaping bladder in the vulcanization-shaping, it is favorable that the edge of the dividing face between the mutually adjoining side ring pieces located at the side of the tire shaping face is positioned in a place corresponding to an outward corner part in the radial direction at a side delaying the enlarging deformation of the uncured tire among corner parts adjoining to each other inward and outward in the radial direction in the depressing portion forming the bead guard.
A vulcanization mold usable for carrying out the manufacturing method according to the invention is a vulcanization split type mold comprising a pair of annular side rings forming a pair of bead portions and a pair of sidewall portions of a pneumatic tire, many split segments for the formation of a tread portion of the pneumatic tire each freely going in and out inward and outward in a radial direction with respect to a center axis line of the side ring and engaging with the respective side ring and a depressing portion formed in at least one side ring for the formation of a bead guard protruding outward from the pneumatic tire in the bead portion, and is characterized in that the side ring is constructed with two or more side ring pieces annularly divided inward and outward in a radial direction with respect to a center axis line of the split type mold, and the depressing portion is formed in at least two adjoining side ring pieces, and an edge of a dividing face between the mutually adjoining side ring pieces located at a side of a tire shaping face is positioned in the depressing portion, preferably a bottom of the depressing portion.
In a preferable embodiment of such a mold, the edge of the dividing face between the mutually adjoining side ring pieces disposed in the depressing portion and located at the side of the tire shaping face is positioned in a corner part at the bottom of the depressing portion.
When plural corner parts are existent in the bottom of the depressing portion forming the bead guard, it is favorable that the edge of the dividing face between the mutually adjoining side ring pieces forming the depressing portion and located at the side of the tire shaping face is positioned in a deepest bottom corner part among the plural bottom corner parts, or that the edge of the dividing face between the mutually adjoining side ring pieces forming the depressing portion and located at the side of the tire shaping face is positioned in an outward bottom corner part in the radial direction among the plural bottom corner parts separated away from each other in the radial direction of the depressing portion.
Thus, as previously mentioned, ventilation can be smoothly and surely realized from such a most easily air reserving portion that the uncured tire most lately contacts with the tire shaping face of the mold.
Preferably, each of the side rings is divided into three or more side ring pieces.
More preferably, a chamfered portion specifying a position of the edge of the dividing face located at the side of the tire shaping face is formed in at least one of the mutually adjoining side ring pieces.
Further, it is favorable that the side ring pieces in each of the side rings have a structure capable of assembling and dissembling them and are integrally united with each other in a vulcanization work including open-close of a vulcanizing machine provided with the mold.